Two stages of coking to make a high quality coke

ABSTRACT

The coking process disclosed herein provides a method for producing high quality petroleum coke which is particularly suitable for use in manufacturing electrothermic graphite electrodes. A high-boiling gas oil fraction is obtained from the overhead products produced during the coking of a petroleum oil residuum. This high-boiling gas oil fraction is segregated during the manufacture of regular or average quality petroleum coke and is subsequently coked in a separate coking operation under delayed coking conditions of time, temperature and pressure to produce a superior quality petroleum coke.

United States Patent Inventor Harvey E. Keel Johnson City, Tenn.

Appl. No. 829,082

Filed May 29, 1969 Patented Nov. 2, 1971 Assignee Great Lakes Carbon Corporation New York, N.Y.

TWO STAGES OF COKING TO MAKE A HIGH 1 References Cited UNlTED STATES PATENTS 2,775,549 l2/l956 Shea,.|r. 196/52 2,922,755 H1960 Hackley,.lr 208/39 Primary Examiner-Herbert Levine Attorney-Cari F. Peters ABSTRACT: The coking process disclosed herein provides a method for producing high quality petroleum coke which is particularly suitable for use in manufacturing electrothermic ga s? m graphite electrodes. A high-boiling gas oil fraction is obtained g from the overhead products produced during the coking of a U.S.Cl 208/50, petroleum oil residuum. This high-boiling gas oil fraction is 208/53,208/i3i segregated during the manufacture of regular or average lnt.Cl ..Cb55l00, quality petroleum coke and is subsequently coked in a ClOg 37/00 separate coking operation under delayed coking conditions of Field of Search 208/50, 53. time, temperature and pressure to produce a superior quality l3i petroleum coke.

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PATENTEDHUY 2 1911 3,617, 480

V 33 FRACT. TOWER v STRIPPING 34 32 TOWER CRUDE 0R IV LENT 27 24 Lil T Q STORAGE FQRNACE INVI'IN'I'OR.

HARVEY E. KEEL TWO STAGES OF COKING TO MAKE A HIGH QUALITY COKE BACKGROUND OF THE INVENTION This invention relates to a method of producing a high quality petroleum coke which, when used as an aggregate in the manufacture of electrothermic graphite electrodes, will 'produce electrodes Process," a low coefficient of thermal expansion (CTE), improved electrical resistivity and other properties which will enhance the performance of large diameter graphite electrodes in electric steel furnaces. Petroleum coke is usually produced by coking a high-boiling petroleum residuum such as a reduced or topped crude oil or vacuum reduced crude oil, in a variety of coking methods. One of the most common methods employed currently is the so-called delayed coking" process which is described in an article by R. .l. Diwoky Continuous Coking of Residuum by the Delayed Coking Process", in Refiner and Natural Gasoline Manufacturer, Vol. 17, No. II, Nov. 1938. The present invention in volves the use of this type of coking operation.

The manufacture of high quality or "premium" petroleum coke suitable for use as an aggregate raw material for the manufacture of graphite electrodes involves the selection of certain selected petroleum-derived feedstocks which are used as the feed to the delayed coking process. One of the earliest disclosures of a process for producing this type of coke appears in U.S. Pat. No. 2,775,549-Shea. The two most economic and commerciallyfeasible methods for producing coke in accordance with that process are described in examples VIII and IX of the patent which teach the use of a gas oil fraction obtained from a Mid-Continent asphaltenic or naphthenic crude oil as a feedstock for catalytic and/or thermal cracking and recovery of high-boiling bottom fractions" or tars from these operations which are then coked under carefully controlled conditions in the delayed coking process. A similar and equivalent process is disclosed in U.S. Pat. No. 2,922,755 which also describes the differences between regular petroleum coke produced from petroleum residues and premium" coke. Another process involving the utilization of overhead products from the delayed coking petroleum residues is disclosed as U.S. Pat. No. 3,089,074. A distillate fraction derived from the delayed coker overhead is thermally polymerized and is then combined with the residuum feed which is then sent to the delayed coking operation. While this type of operation is designed to increase the coke yield of the original feedstock, it will have little effect, if any, upon the ultimate quality of the coke produced by this process.

OBJECTS OF THE INVENTION It is an object of the present invention to provide a method for producing a high quality or premium" coke from petroleum fractions which previously have not been utilized solely as a feedstock to the delayed coking process. It is a further object of the invention to provide a method for increasing the commercially available supply of high quality petroleum coke. It is an additional object of the invention to produce high quality petroleum coke by a "blocked out" delayed coking operation employing as a feedstock a heavy gas oil fraction derived from a petroleum residuum which itself will not produce premium coke.

SUMMARY OF THE INVENTION The present invention consists of a method for producing high quality petroleum coke from a petroleum residuum, such as a topped or reduced asphaltenic or naphthenic crude oil or vacuum-reduced crude oil of this type which, in and of itself, will not produce high quality or premium petroleum coke suitable for use as a carbon aggregate in the manufacture of large diameter electrothermic graphite electrodes. TI-le petroleum residuum is first heated in a suitable heat exchanger to a coking temperature, after which it is coked under conditions of delayed time, temperature and pressure, to produce coke and distillate products. These distillate products are fractionated and there is recovered therefrom a heavy gas oil fraction at least percent of which has a boiling point above 700 F. This heavy gas oil fraction is then coked in a separate or blocked out" coking operation under delayed coking conditions of time, uniform temperature and pressure. After the coking drum or vessel is about two-thirds full of coke, the feed to the coking vessel is stopped and switched to another vessel to continue the operation and the coke is removed from the first vessel by any of a variety of procedures, such as hydraulic decoking, which are well-known to those skilled in the petroleum refinery art. The resulting coke is then calcined at a temperature of up to 2,600 F. to reduce its volatile content to less than 0.5 percent by weight. The calcined material is then crushed and sized and is an excellent material when employed as a carbon aggregate in the manufacture of high quality electrothermic graphite electrodes which are used in electric steel furnaces.

Sample electrodes prepared for CTE determination by the method disclosed in U.S. Pat. No. 2,775,549 (except for calcination of the coke to 2,280 F.) have a CTE (measured over the temperature range 32 to 167 F.) of 6.0 XIO'7 or less and the coke will produce large electrothermie electrodes which have advantageously low CTE and electrical resistivity properties.

DETAILED DESCRIPTION OF THE DRAWING The foregoing objects of the invention and summary thereof may be further illustrated by a detailed description of the invention as supplemented by the drawing which depicts schematically a flow diagram of a preferred embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT A reduced or topped asphaltenic or naphthenic petroleum oil, or vacuum reduced oil of this type is conveyed from the crude oil tower (not shown), from which it leaves at a temperature of about 600 F., through line 21 and valves 31 and 40 (valve 36 being closed) into the bottom portion of a fractionating tower 10 which is also receiving hot overhead or distillate products from the coking vessel as later described herein. The hot reduced crude oil together with small amounts of high-boiling materials which are not distilled overhead from the fractionating tower are conveyed through line 22 and valve 41 to a pump I1 and thence through line 23 to a heat exchange furnace 12 of conventional design. The feed conveyed to the coker from the furnace through line 24 is at a temperature within the range of 900 to 950 F. By means of a valve, the feedstock is conveyed into coke drums 13 or by line 25 or 250. Two-coke drums are normally employed in refineries using the delayed coking process because while one drum is being filled with the hot residuum and producing coke and overhead products, the other drum is being cooled and the coke removed therefrom by hydraulic or other means well-known to those skilled in the art. Tl-Ie hot feed entering the coke drum undergoes decomposition (cracking), polymerization and aromatization. As these chemical processes proceed to completion, petroleum coke is formed and gradually fills the coke drum while the distillate products produced by these chemical reactions are conveyed out of the drum. With valve 34 closed and valve 32 open, these distillate products are conveyed to a stripping tower 14 where a highboiling gas oil faction, at least 80 percent of which has a boiling point (atmospheric conditions) above 700 F is collected. If required, the stripping tower 14 may be equipped with a reboiler or jacketed for recirculating of heat exchange media necessary to maintain the desired bottom temperature. Also, a partial condenser can be used as a reflux for maintaining the top stripping temperature. The lighter boiling products from the stripping tower are conveyed through valve 33 and line 28 to the fractionating tower 10 from which the lower boiling products including gas, gasoline and light gas oil are recovered. The high-boiling gas oil faction which collects in the stripping tower 14 is conveyed through line 27 and valve 19 to a heat exchanger 44 into a storage tank 20 to be processed in a separate or blocked out" delayed coking operation.

If it is not desired to admit the heated reduced crude oil or equivalent into the bottom portion of the fractionating tower 10, this unit may be bypassed by closing valve 40 and opening valve 36 and pumping the reduced crude, through a heat exchanger (not shown) if necessary, through line 35, pump 1 l and line 23 to the heat exchange furnace 12.

To produce high quality or premium petroleum coke according to this invention from the high-boiling gas oil fraction which has been collected in storage tank 20, the reduced crude oil or equivalent feed is shut off by valve 31 and the gas oil fraction is conveyed through line 29 and valve 30 to a heat exchanger 37 and thence either to the lower zone of fractionating tower through valve 38 and line 42 or, altematively, bypassing the fractionating tower by closing valve 38 and opening valve 39 and conveying it through line 43 to pump 11 and through line 23 to the heat exchange furnace 12. The heavy gas oil fraction is uniformly heated in the furnace to the temperature up to 950 F., preferably 925 to 945 F., and then conveyed to one of the coke drums 13 or 13a through line 24. The heavy gas oil is passed into the coke drum under uniform temperature and pressure conditions, where it undergoes decomposition (cracking), polymerization, aromatization and ultimately coke formation. The lower boiling products produced from these reactions leave the top of the coke drum and are returned to the fractionating tower through open valve 34 and line 28. in this operation, the stripping tower 14 may be omitted from the circuit and valves 32. 33 and 19 will be closed. In the fractionating tower most of the cracked distillate products will be fractionated as overhead products and some higher boiling material will be recycled to the coke drum through line 22 and valve 41. After the coke drum is about two-thirds filled with coke, the valve between the drums is switched and the feed is admitted to the second drum while the first drum is being cooled internally by high temperature, high pressure steam after which the coke is removed from the drum preferably by hydraulic means which are well-known to those skilled in the art. The coke yield, based on the fresh feed from tank 20, will be within the range of to 35 percent, depending on the coking cycle. When a sample of this coke is calcined at a temperature sufficiently high so that the residual volatile matter is about 0.1 percent and the coke is ground to flour size (52 percent --200 mesh) and is used to make test graphite electrodes, the resulting electrodes will have a CTE of about 4.0 l0 (over the temperature range 32 to 167 F.) and is an excellent material to produce carbon aggregate in the manufacture of high quality large electrothermic graphite electrodes.

If the refining operation does not contemplate the use of a stripping tower 14 to collect the heavy gas oil fraction which is used as the feedstock to produce premium coke in accordance with this invention, the entire cracked, distillate product from coking the reduced crude oil or equivalent can be sent to the fractionating tower 10 which is operated at a bottom temperature sufiiciently high so that at least percent of the heavy gas oil which collects at the bottom of the unit has a boiling point of about 700 F. in such case, the reduced crude or equivalent will bypass the fractionating tower l0 and, with valves 40 and 41 closed and valves 31 and 36 open, will be sent directly to the furnace 12 and thence to one of the coke drums. This material can then be sent to storage tank 20 by closing valve 41 and opening valve 45 and conveying the material through line 46.

While I have described my invention specifically in terms of the foregoing preferred embodiment, it should be understood that it is limited only by the scope of the appended claims.

What is claimed is: 1. In a process for producing high quality petroleum coke from a petroleum residuum which will not produce said coke directly which comprises removing from the residuum those components which readily form an insoluble phase upon heating the residuum 650-l ,000 F., coking the remaining tar in a quiescent pool, and recovering a high quality petroleum coke therefrom; the improvements consisting of the following combination:

a. heating said residuum to a coking temperature;

b. coking said heated residuum under conditions of delayed time, temperature, and pressure to produce coke and distillate products;

c. fractionating said distillate products to recover therefrom gas, gasoline, light gas oil, and a heavy gas oil fraction at least 80 percent of which has a boiling point about 700 F.; and

d. coking the heavy gas oil fraction from step (c) in a subsequent and separate coking operation under delayed coking conditions at a uniform temperature between 925 to 950 F. and underpressure.

2. A method according to claim 1 wherein said petroleum residuum is derived from a topped or reduced asphaltenic or naphthenic crude oil.

3. A method according to claim 1 wherein said petroleum residuum is derived from a vacuum reduced asphaltenic or naphtheniccrude oil.

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2. A method according to claim 1 wherein said petroleum residuum is derived from a topped or reduced asphaltenic or naphthenic crude oil.
 3. A method according to claim 1 wherein said petroleum residuum is derived from a vacuum reduced asphaltenic or naphthenic crude oil. 